It has emerged that Lanza, 20, had shown no known propensity for violence.

Reports suggested Nancy Lanza, 52, was a gun enthusiast who might have kept weapons for defence.

Lanza, dressed in black, killed his mother in her bed at the stately two-storey home they shared before driving 3km to the school he had attended as a child.

There, he gunned down first-grade students, carers and teachers before turning a firearm on himself.

Among the 26 dead were six women, including principal Dawn Hochsprung and psychologist Mary Sherlach, who tried to intervene. According to one account, Ms Hochsprung lunged at Lanza as he neared a classroom.

Lanza gunned down both women before forcing his way into classrooms and shooting children and their teachers, many at close range.

"She is doing fine, she is being treated and she will be instrumental in this investigation, as you can understand," Lt Vance said.

Newtown police said they uncovered "very good evidence" of motive - apparently from Lanza's home.

Lanza's father, Peter, said his son's actions were beyond comprehension.

"Our hearts go out to the family and friends who lost loved ones and to all those who were injured," he said.

"Our family is grieving along with all those who have been affected by this enormous tragedy.

"No words can truly express how heartbroken we are. We are in a state of disbelief in trying to find whatever answers we can.

"We, too, are asking why.

"Like so many of you, we are saddened and struggling to make sense of what has transpired."

A picture began to emerge of Lanza's troubled adolescence. The skinny, mop-haired loner struggled to relate to other students or teachers at Newtown High School. He is believed to have suffered from Asperger's syndrome, an autism spectrum disorder marked by social dysfunction.

One school supervisor suggested Lanza seemed not to feel physical or emotional pain. Others described a "tech-geek" who "never really talked at all".

Richard Novia, the school district's former head of security, said Lanza clearly "had some disabilities" and appeared detached.

"If that boy would've burned himself, he would not have known it or felt it physically," Mr Novia said.

He said he met school guidance counsellors and Nancy Lanza to understand the boy's problems.

"He would have an episode, and she'd have to return or come to the high school and deal with it," Mr Novia said. He said Lanza would sometimes withdraw completely "from whatever he was supposed to be doing".

"You had yourself a very scared young boy, who was very nervous around people he could trust, or he refused to speak with."

Lanza was not physically bullied, though he may have been teased, Mr Novia said. He said Lanza had never presented as a threat.

"Somewhere along in the last four years, there were significant changes that led to what has happened," he said. "I could never have foreseen him doing that."

Former classmate Kateleen Foy said Lanza was "painfully shy" but showed no aggression: "I want people to know he wasn't always a monster."

Police who searched the 115 sq m, $500,000 Lanza home said there was evidence he had an interest in violent video games.

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